The state of the art in power converters provides an adjustable voltage and frequency to the output through a pulse width modulated (PWM, pulse width modulation) voltage source inverter drive. Power converters can be used in uninterruptible power supplies (UPS), electric motors, etc. The PWM command is used in the power converter for controlling power to electrical devices, made practical by modern electronic power switches. The duty cycle of a switch (ratio of on-time to total cycle time) is varied to achieve a desired average output voltage, current etc., when averaged over time.
A typical power converter is a switching power converter. It has two or more power semiconductor devices such as power semiconductor switches. The power semiconductor switches can, for example, be implemented by insulated-gate bipolar transistors (IGBT). A simple logical error—turning a transistor on at the wrong instant—can cause catastrophic failure in the right or wrong circumstances. When the state of the transistor must change due to requested PWM, the conducting transistor is turned off. Then, after a delay (called dead time), the other transistor is turned on. The delay is added to ensure that there is no possibility of both transistors conducting at the same time. Dead time is necessary to prevent short circuit of the power supply of the switching power converter.
Although the dead time is short, it causes deviation from the desired output. The deviations result in reduced output voltage, distorted machine currents, and torque pulsations. Dead time in the switching output stage also causes nonlinearity in the power circuit transfer function that may be difficult to remove.
Due to the dead time, it is also never possible to reach maximum modulation depth. The maximum actual duty cycle of PWM is limited by the dead time between the power converter commands. Dead time limits the time that the power converter can spend in an active state as a fraction of the total time under consideration. System requirements of the power converter for the minimum allowed command pulse also limits the duty cycle. Due to these constraints, the capacity of the power converter is not fully used. For example, an inverter having a 15 kHz switching frequency, 2 μS dead time, and 2 μS minimum pulse can reach a maximal modulation depth of only 94%. 6% of the theoretical modulation range is lost, which translates into a 6% loss of efficiency. This causes loss in maximum reachable output voltage mostly and additionally negative effect on output voltage waveform in closed loop system, for example due to an earlier saturation of the PWM command.
A known solution to improve the loss and performance has been to increase the DC bus voltage to compensate loss of modulation depth. This, however, increases the cost of the power converter system. The present invention is directed to overcoming one or more of the problems as set forth above.